The present invention relates to an image forming apparatus in which an original document is placed on a platen, the apparatus having a moving exposure lamp, a plurality of reflection mirrors, and a fixed focal length lens, a document image being formed on an image carrier, and specifically relates to an optical mechanism for image magnification change.
In an image forming apparatus, specifically, an exposing apparatus which exposes the document is provided with: an exposing lamp which exposes the document from the lower surface of the platen; a first scanner unit into which the first reflection mirror is assembled; a plurality of reflection mirrors which receive the reflected light from the first reflection mirror, and form an image with an image forming lens; and a final reflection mirror which sends the document image from the image forming lens to the image carrier. When the image forming lens and the final reflection mirror are moved in the direction of the optical axis of the image forming lens, an enlargement or reduction operation of the document image is conducted in the image forming apparatus. In the operation for selecting enlargement or reduction of the document, since a movement amount of the image forming lens and that of the final reflection mirror are different from each other, the following publications are widely known as a means in which a lens cam member is provided into a frame of the image forming lens, or a means in which image is formed by a lens cam member which is interlocked with the frame of the image forming lens: Japanese Patent Publication Open to Public Inspection Nos. 195260/1982 and 182431/1984, and Japanese Utility Model Publication Nos. 112239/1985 and 70844/1986.
As described above, in a means, in which the lens cam member is provided in a lens unit in which the image forming lens is housed, and the reflection mirror is moved through an operating member so that an electrostatic latent image is formed onto the image carrier, since an error of the focal length (normally, .+-.1%) produced in the manufacturing process can not be allowed, the following methods have been adapted: a method in which various lens cam members having different surface inclination are prepared, and the lens cam member having the smallest error is used; and a method in which the lens cam member is divided into an enlargement portion and a reduction portion and is adjusted.
As described above, the cam surface for enlargement and that for reduction are formed in the lens cam member provided in the lens unit. The cam surfaces for enlargement and for reduction are formed on the one surface of the lens cam member including the cam surface for reference copying, the cam surfaces are contacted with a portion of the operation member, and the lens unit is moved and also the lens cam member is moved. In this connection, for the purpose that the entire cam surfaces for enlargement and for reduction act on a portion of the operation member, the lens cam member becomes large, and it is necessary that the lens cam member is largely moved. Accordingly, it is necessary that a large space for operation is formed corresponding to the movement distance in an exposure section, so that the entire apparatus becomes large, which is disadvantageous.
Further, as described above, in order to reduce the error of the focal length in the manufacturing process as small as possible, it is necessary that various lens cam members, in which cam surfaces for enlargement and for reduction are different from each other, are prepared. Accordingly, the manufacturing cost is increased, and a long period of time is necessary in order to select the lens cam member for correcting the error of the focal length, which are largely disadvantageous.